A tire has a structure that various materials, such as rubber, chemical fibers, and steel cords, are layered. If there is a non-uniform portion in the layered structure, a projection (convex portion), called a “bulge”, or a recess (concave portion), called a “dent” or a “depression”, generates in a region where resistance against pressure is relatively weak, when the tire is filled with air. The tire having such a shape defect, i.e., the bulge or the dent, needs to be excluded from shipped products through inspection in consideration of problems regarding safety and an external appearance.
Usually, in the tire shape inspection, the tire is rotated by a rotating machine in a state that a predetermined displacement sensor is positioned to face a tire surface (e.g., a sidewall surface or a tread surface).
For example, Patent Literature (PTL) 1 discloses a technique of irradiating a surface of a rotating tire with a slit light (line light), picking up an image of the slit light, and performing shape detection using an optical cutting (light-section) method based on the picked-up image, to thereby detect the tire surface height. With the disclosed technique, distribution information of surface height measurement values is obtained for many positions over a circumferential range of 360° of the tire surface. The surface height measurement value for each of the positions over the circumferential range of 360° of the sidewall surface or the tread surface is plotted in a two-dimensional coordinate system in which a first coordinate axis (e.g., an X-axis) represents the radial direction of the tire and a second coordinate axis (e.g., a Y-axis) represents the circumferential direction of the tire. The information thus obtained is called “surface height distribution information” hereinafter. Assuming that the surface height measurement value corresponds to a brightness value of each pixel in image data, therefore, the surface height distribution information can be handled in a similar manner to monochrome image data on a computer (image processing apparatus).
In the tire shape inspection, a shape defect inspection process for the sidewall surface is executed based on the surface height distribution information.
Embossed marks indicating the product type/model, the size, the maker's logo, etc. (called “indication marks” hereinafter) are formed on the sidewall surface of the tire. In the shape defect inspection process for the sidewall surface, therefore, embossed portions of the indication marks require to be made not erroneously detected as shape defects.
In the known shape defect inspection process, it is often performed to determine whether height change in the circumferential direction of the tire is within an allowable range, based on data obtained through low-pass filtering of the measured values of one line in the circumferential direction of the tire, which are included in the surface height distribution information.
For example, PTL 2 describes an example in which the above-mentioned low-pass filtering is realized by excluding high-frequency components from data obtained with a fast Fourier transform process that is executed on the measured values of one line in the circumferential direction of the tire, and by executing an inverse Fourier transform process on the remaining data (see paragraph [0003] in PTL 2).
Further, PTL 3 describes a technique for realizing the low-pass filtering by, instead of the fast Fourier transform process, a method of calculating contact points between the measured values of one line in the circumferential direction of the tire and a parabolic curve, and executing straight-line (linear) interpolation between two calculated contact points.
Moreover, PTL 2 describes a technique of detecting, from among data obtained with smoothing differentiation of the measured values of one line in the circumferential direction of the tire, a range from a position where change of the measured value rises steeply to a position where change of the measured value falls steeply, as a range where the indication marks exist, and excluding the detected range from an inspection target.
The above-described known techniques are premised on the fact that, in the circumferential direction of the tire, change of the surface height in a portion including the shape defect to be detected is comparatively gentle, while change of the surface height in a portion including the indication marks is steep.